In the conventional fluid volume measuring apparatus according to U.S. Pat. No. 5,212,979 (JP-A-H4-255568), fluid level vertically displaces in a grass tube in accordance with a volume of fluid flowing out of an object to be measured (measured object). The fluid level is detected using a line camera to measure a volume of fluid flowing out of the measured object. In the conventional flow volume measuring apparatus according to JP-A-10-9991, variation of fluid level in a leakage-detecting pipe is detected as variation of pressure, so that a volume of fluid flowing out of the measured object is detected.
However, fluid level on the opening side of the grass tube or the leakage-detecting pipe needs to be completely returned to a standard position to measure a volume of the fluid in the above conventional structure. Fluid adheres to the inner wall face of the tube (pipe) when fluid level vertically downwardly displaces to the standard position in the tube. Therefore, the fluid level becomes unstable before the fluid, which adheres to the inner wall face of the tube, completely returns downward.
When measurement of the fluid level is started before the fluid adhering to the inner wall face of the tube completely returns downward, the fluid volume cannot be precisely measured. Accordingly, measurement of fluid volume needs to be halted until the fluid adhering to the inner wall face completely returns downward for precise measurement of fluid volume in the conventional structures. As a result, an interval between measurements is lengthened.